In recent years, there have been made various kinds of proposals concerning a dynamic pressure bearing apparatus used to support various kinds of rotors in a device having a portion which rotates at a high speed, which is typified by a magnetic disc, a polygon mirror or an optical disc. For example, in a dynamic pressure bearing apparatus adopted in a hard disk drive apparatus (which will be referred to as an HDD hereinafter) shown in FIG. 21, a rotary shaft (shaft) 2 is rotatably inserted in a fixed bearing sleeve (bearing) 1, and a lubricating fluid F such as an oil or a magnetic fluid is injected into a minute gap between an inner peripheral surface of the bearing sleeve 1 and an outer peripheral surface of the rotary shaft 2, thereby constituting two radial bearing portions RB and RB provided at respective positions so as to be separated from each other in the axial direction. It is to be noted that, in this HDD, a rotary hub 5 used to hold a recording disk (not shown) is joined to a part of the rotary shaft 2 which is protruding from the bearing sleeve 1 by press fitting or shrink fitting, and the recording disk is held by a clamper (not shown) screwed by utilizing a screw hole 7 at an upper end portion of the rotary hub 5.
In such a dynamic pressure bearing apparatus, a thrust plate 3 is provided at least one end of the rotary shaft 2 as well as the above-described radial bearing portions RB and RB, and thrust bearing portions SBa and SBb are constituted between the bearing sleeve 1 and a counter plate 4 attached to the bearing sleeve 1. The lubricating fluid F is continuously injected into the thrust bearing portions SBa and SBb from the radial bearing portions RB.
Here, as to attachment of the thrust plate 3 to the rotary shaft 2, the structure that joining is performed by a fastening force of such a headed screw 6 as shown in FIG. 21 is conventionally known from, e.g., Japanese Registered Utility Model Publication No. 256791, Japanese patent Application Laid-open Publication No. 74841/1996 and others. Such a screwing structure holds the thrust plate 3 so as to be sandwiched in the axial direction between a head portion 6a of a headed screw 6 screwed at a lower end portion of the rotary shaft 2 and an end surface of the rotary shaft 2.
The screwing structure of this thrust plate 3 corresponds to a request or the like for a reduction in thickness of the dynamic pressure bearing apparatus. That is, when a height of the dynamic pressure bearing apparatus in the axial direction is greatly suppressed owing to a reduction in thickness of the dynamic pressure bearing apparatus, a joint length of the thrust plate 3 relative to the rotary shaft 2 in the axial direction is reduced, and the joint strength of the thrust plate 3 may be possibly lowered. In particular, in an apparatus such as a mobile computer which is presupposed to be carried, the impact resistance at the time of falling or the like must be sufficiently assured, and hence the joint strength of the thrust plate 3 is a serious problem. Thus, adoption of the above-described screwing structure is used to firmly fix the thrust plate 3.
Further, as another attachment method of the thrust plate 3 relative to the rotary shaft 2, there is a fixing technique such as press fitting or shrink fitting as shown in FIG. 22, for example. In this case, the thrust bearing portions SBa and SBb are configured above and below the thrust plate 3′ between the thrust plate 3′ fixed at the lower end portion of the rotary shaft 2′ by press fitting or shrink fitting and the bearing sleeve 1 and between the thrust plate 3′ and the counter plate 4′ attached to the bearing sleeve 1. It is to be noted that a rotary hub 5′ used to hold the recording disk is joined to a part of the rotary shaft 2′ which is protruding from the bearing sleeve 1 by press fitting, shrink fitting or the like, and a damper 9 is fixed at the upper end portion of the rotary shaft 2′ by the headed screw 6′ so that the recording disk 8 is held by a pressing force of the damper 9 in the axial direction. It is to be noted that the reference numeral 6a′ denotes a screw head portion, 6b′ designates a male screw portion and 2a′ denotes a female screw portion formed from the upper end surface of the rotary shaft 2′ toward the lower end surface of the same in the drawing.
In the fixing structure of the thrust plate 3 using screwing shown in FIG. 21, however, the lubricating fluid F gradually penetrates by the capillarity generated in a gap, to the gap of the screw portion where the female screw portion formed to the shaft 2 and the male screw portion on the headed screw 6 side are screwed. As a result, the lubricating fluid F in the thrust dynamic pressure bearing portion is substantially decreased and becomes insufficient, thereby possibly reducing the duration of life of the bearing.
Furthermore, since a hole which is preparedly provided for thread cutting is opened longer than the female screw portion, a space (which is called a screw non-occupied space) is generated between the end of the headed screw 6 and the bottom of the screw hole. Since air remains in the screw non-occupied space, when the remaining air is expanded by an increase in temperature, the expanded air pushes the lubricating fluid F toward the outside, which results in the leak. Such leak of the lubricating fluid F may possibly lead to a fatal problem in the HDD or the like requiring the cleanness.
In the dynamic pressure bearing apparatus shown in FIG. 22, the female screw portion 2a′ used to fix the damper 9 formed to the rotary shaft 2′ is configured so as to be cut by a bottomed hole. Therefore, foreign matters such as chips generated at the time of thread cutting or the like are readily deposited on the bottom of the hole in which the female screw portion 2a′ is cut. Moreover, the foreign matters once deposited on the bottom of the hole of the female screw portion 2a′ cannot be completely removed even if a cleansing step or the like is carried out. If these foreign matters spatter to the outside during use of the apparatus, they adhere to the recording disk which must be cleaned, which may possibly result in a fatal error problem such that recording/reproduction of information is disabled, for example.
Additionally, if a reduction in thickness of the apparatus further advances, a length of the lower hole of the female screw portion 2a′ formed to the above-described rotary shaft 2′ is also decreased. In such a case, the female screw portion 2a′ is subjected to thread cutting in the shortened lower hole by using the similarly shortened threading tap tool. However, by doing so, a large stress is loaded to the tap tool when shaping the screw, a problem such that the tap tool is readily bent occurs, thereby possibly lowering the productivity.
It is an object of the present invention to provide a dynamic pressure bearing apparatus which can readily increase the joint strength of the thrust plate relative to the shaft and simultaneously obtain the excellent dynamic pressure characteristic and high productivity. It is another object of the present invention to provide a dynamic pressure bearing apparatus excellent in the cleanliness and the processability.